Electron discharge device



aw. W

June 21, 1960 Filed June 16, 1959 w. F. NIKLAS 2,941,857

ELECTRON DISCHARGE DEVICE 2 Sheets-Sheet 1 INVENTOI? Wilfrid F 77ZZa5 ATTORNEY June 21, 1960 W. F. NIKLAS ELECTRON DISCHARGE DEVICE} Filed June 16, 1959 ASSEMBLE ALL COMPONENTS OF IMAGE CONVERTER EXCEPT MULTl-LAYER PICK UP SCREEN AND SEAL IN ENVELOPE.

SEAL SUBASSEMBLY UNTO A VACUUM SYSTEM AND EVACUATE TO A PREDETER- MINED PRESSURE.

TURN OFF HEAT, COOL TUBE AND TIP OFF.

Ff'a. 2

STEP STEP I la STEP II:

STEP IIIu STEP III STEP III

2 Sheets-Sheet 2 HEAT MULTl-LAYER PICKUP SCREEN WITHOUT ANTIMONY LAYER TO HIGHEST TEMPER- ATURE COMPATABLE WITH MATERIALS COMPRISING THE SCREEN.

PUT PARTIALLY PREPARED PICKUP SCREEN INTO PUMP STATION AT TEMPERATURE ABOVE IOO CENTIGRADE AND EVAPORATE ANTIMONY LAYER.

STEP

PREPARED PICKUP SCREEN AND RESEAL ENVELOPE.

EVACUATE ASSEMBLED ENVE- LOPE AND HEAT TO LESS THAN SUBLIMATION TEMPERATURE OF ANTIMONY AGAINST A A MAXIMUM PRESSURE.

STEP

CESIATE ANTIMONY LAY ER BY INVENTOR United States 1 2,941,857 ELECTRON DISQIHARGE DEVICE Wilfrid F. Niklas, Chicago, ilk, assignor to The Rauland (Iorporation, a corporation of Hlinois Filed June 16, 1959, Ser. No. 820,636

5 Claims. (Cl. 316-21) This invention relates to electron discharge devices and more particularly to a novel method of manufacturing an electron-optical image converter.

It is well understood in the art of image converters and intensifiers that an electron image-may be projected from a photoemissive cathode in response to the excitation of incident radiation, whether visible or invisible, representing a source or object image. The photoemissive cathode, responding to the incident radiation, generates an electron image which may be directed by the means of a suitable electric field to a fluorescent viewing screen where the electron image is converted to a visible reproduction constituting a replica of the original image projected on the photoemissive cathode.

The photoernissive cathode is usually one portion of a multi-layer pickup screen. Where the pickup screen is of the multi-layer type, it generally comprises a phosphor layer and a photoemissive layer applied to a supporting member with a light transparent barrier layer intermediate the phosphor and the photoemissive layers. The phosphor layer may consist of a silicon resin zinc sulfide phosphor mixture applied to a supporting member, while the photoemissive. layer is usually of the cesiated antimony type. The intermediate barrier layer is generally a light transparent material such as a thin layer of aluminum oxide and is necessary to protect the phos-' phor-resin surface from the catalytic action of the'cesiurn arising during the processing of the cesium antimony photocathode. I

In the manufacture of image converters using cesiated antimony photoemissive cathodes, it is necessary that the photoemissive layer be of a type known as a semi-transparent photocathode; thatis to say, a surface upon which light quanta may impinge and cause the opposite surface thereof to emit an electron image of the impinginglight image. 7

Existing methods of manufacturing image converters generally entail making an assembly of all components of the image converter, including a partially processed multi-layer pickup screen, and sealing their enclosing envelope preparatory to evacuating. Ordinarily, the pickup screen at this juncture comprises the supporting member, the phosphor-resin layer, andthe barrier layer. The further processing of the screen required to form the photoemissive layer is generally accomplished by first evaporating a layer of antimony of the required thickness over the barrier layer while the pickup screen is assembled and mounted within the tube envelope. the antimony has been thus deposited onto the barrier layer, the antimony surface is cesiated in a manner wellknown in the art.

One reason for this approach, as distinguished from forming the antimony surface externally rather than in situ, is to minimize the possibility of antimony oxide accumulating on the antimony layer. If such oxidation occurs, there is a reduction of the amount of antimony otherwise available for the formation. of the photo.- emissive surface. Moreover, the presence of antimony After I oxide increases the surface resistance of the antimony layer and both of these efiects contribute to a substantial reduction of the photosensitivity of the finished cesiated antimony layer.

Many difiiculties attend this fabricating procedure; in particular, the difiiculty of controlling the deposit of the antimony and confining it to the substrate. The deposition of small amounts of antimony on various internal surfaces of the tube other than the desired photocathode. area is unavoidable and the subsequent cesiation renders these other surfaces photoemissive. As a consequence, the contrast ofthe converter-is very adversely aifected and the high voltage breakdown strength of the tube is undesirably reduced.

Further, in the manufacture of large image converters the evaporation of the antimony layer in situ becomes complicated due to space limitations of the device itself, to the antimony layer being of uneven thickness, to the speed of evaporation of antimony, and to numerous other factors. Additionally, in situ evaporation of antimony makes it impossible to subjectively inspect the antimony base layer for imperfections deleterious to the tube performance.

All previous attempts to accomplish the evaporation of antimony external to image converter tubes utilizing semi-transparent photocathodes have met with failure due to the extreme susceptibility of the antimony layer to oxidation which results in imperfect operation of the device.

It is therefore an object of this invention to provide a novel method of manufacturing an image converter tube. it he furtherfobject of this invention to provide a novel method of producing an image converter including a backsurface type photocathode featuring an externally evaporated antimony surface.

It is another object of this invention to provide an image converter structure having a cesiated antimony cathode in which objectionable cfiects of migratory traces of antimony are minimized or eliminated.

It is a still further important object of the present invention to provide a novel method of producing image convertersof the type comprising a cesiated antimony photoemissive cathode in such a manner that the proc-' essing of the antimony photoemissive surface is substantially simplified over previous methods.

The present invention concerns a method of manufa'c turing an image converter comprising an evacuated en velope enclosing a plurality of components including a multi-layer pickup screen having a cesiated antimony layer. The method includes the step of making a subassembly of all components of theimage converter except the multi-layer pickup screen-and sealing them in an envelope. The envelope is then evacuated to a predetermined pressure and heated to a minimum tempera ture substantially in excess of the sublimation temperature of the antimony layer.

properties of its constituent'materials. heating, the temperature of the partially prepared screen is again evacuated whilethe temperature is increased to a maximum value less than the sublimation temperature of the antimony. The rate of temperature increase is controlled to maintain the pressure in the envelope at A multi-layer pickup screenis prepared complete except for its antimony layer and the partially prepared screen is heated to a temperature as highas compatible with the physical and chemical After this initial" less than a predetermined maximum and the antimony layer is then cesiated.

The advantages of the invention are especially apparent when embodied in X-ray image converters. Accord ingly, theinvention will be so described, although it will be understoodthat the inventive teachings may also be applied with great advantage to image converters'of other types.

The features of the present invention which are be lieved to be novel are set forth with particularity .in 'the appended claims. The invention together with further objects and advantages thereof may best be understood, however, by reference to the following description taken in connection with the accompanying drawings in which: Figure l is a cross-sectional view of an X-ray image converter embodying the inventionyand f Figure 2 is a flow chart of the methodof manufacturing an X-ray image converter tube.

. The image converter shown in Figure 1 comprises an evacuated envelope having a cylindrically shaped central section, a shoulder portion at one end of the cylindrical portion extending radially inwardly toward the axis thereof and closure portions at the opposite ends of the composite section constituted by the first mentioned portions. More specifically, there is an envelope section which, for most of its length, is substantially a cylinder and this section of the envelope is closed by a re-entrant press 11. The aforementioned shoulder portion of the envelope is that which interconnects the cylindrical section with the re-entrant press. The remainder of the glass envelope comprises a substantially spherical glass section 12 having a diameter approximately equal to that of the cylindrical portion of envelope section 10. Thesections 10 and 12 are pre-sealed around their perimeters to respective metal flanges 13a and 13b which, in turn, are sealed together by heliarc welding. V

Suitably mounted within envelope section 12 is an electron source in the form of a large diameter multilayer pickup screen 14 which approximates a sector of a sphere oriented so that its. concave surface faces toward the re-entrant end of envelope section 10. The pickup screen, for the specific embodiment under consideration, is sensitive to impinging X-rays and to that end includes an X-ray sensitive phosphor layer such as silver activated zinc sulfide or'the like, embedded in a suitable silicon resin 14b andapplied to a spherically shaped aluminum support member 14a. Superimposed on the phosphor layer is the usual .barrier layer 140 whichmay .be of aluminum oxide or. the like and on the surface of which is applied a photoemissive cathode layer 14d. Photo emissive cathode 14d.ma'y be of conventional cesiated antimony layer composition.

A small diameter fluorescent screen'is positioned within the envelope .within the areaencompassed by the shoulder portion thereof and facing the pickup screen. More specifically, re-entrant section 11 is closed by a flat glass plate 15 having transverse dimensionslsmall with respect to thoseof the cathode and bearing on its inner face a suitable fluorescent coating to constitute a viewing screen 15a. Silver activated zinc-cadmium sulfide or the like is the fluorescent material generally employed and the screen is preferably aluminized or otherwise provided with a metallic backing layer 1511. Q

A focusing and accelerating electrode system'is interposed intermediate the photoemissive cathode and viewing screen and it comprisesa focus'electrode 19 adjacent the inner surface-of the envelope and an anode 16 encompassing viewing screen 15a. The focus electrode may conveniently'take the form of a conductive wall coating having one end terminatedat and electrically connected to metal flanges 13a.and 135. If desired, thefocus electrode may be a Wall coating of'copper, aluminum, or the like. structure which is partially cylindrical and partiallylconical in shape terminated at its large end witha skirt por- The anode 16 is a metallic; electrode .4 tion which may be accommodated by mounting over the re-entrant or returned portion of envelope section 11. The opposite end of the anode terminates in a substantially spherical shaped cap 17 having an axial and cireular aperture 18 which provides access for electrons originating at the photoemissive cathode, admitting such electrons for impingement against viewing screen 15a. Like screen 15a, the anode also has small'transverse dimensions relative to those of the cathode. This is usual practice, employing a circular viewing screen having a much smaller diameter than the photocathode in order to obtain a marked intensification of the visible image.

" The convex face of anode cap 17 faces the concave surface of pickup screen 14. The anode encloses viewing screen 15:: and is electrically connected thereto so that the screen is maintained at the same electrical potential as the anode. The structural details of the image converter as thus far described may be entirely conventional and will be familiar to thoseskilled in the art.

Focus electrode 19 in conjunction with anode 16 constitutes an immersion lens for the purpose of focusing electrons originating at the pickup screen through the anode aperture 18 onto the fiuorescent'viewing screen 15a.. In achieving the lens structure, it is desirable to haye the end of electrode 19, which is closest to the anode, overlap the anode cap although it is appreciated that the focus electrode may terminate in line with or even short of the anode cap but these other structures require modification of the focusing potentials in a manner well understood in the art.

In the operation of the described structure, the pickup screen is operated at or near ground potential through a lead extending through envelope section 12 as indicated at 2.4. .The anode structure connects through a lead 25 to a high voltage power supply (not shown) which establishes an anode potential of between 25 and 35 kilovolts. Focusing potential is applied to electrode 19 by connection through. the flanges 13a, 13b and the focus potential is usually in the order of several hundred volts. Having established the operating potentials, an X-ray image may be admitted through end section 12 of the envelope to impinge upon pickup screen 14 and excite phosphor layer 14b. The excitation of the latter produces -a visible image of the impinging X-radiation and the light image traverses transparent barrier layer 140 to excite photoemissive layer 14d. As a consequence, the photoemissive layer emits an electron image having a charge distribution corresponding to the incident light and, therefore, to the original X-ray' image. The focusing and accelerating electrode system causes the electron image to be accelerated, reduced in size and focused on viewing screen 15a through anode aperture 18. Focusing of the electron image on the viewing screen resultsin a visible image and is. highly intensified compared with the equivalent X-ray imageon a standard fluoroscopic screen. V

The improvement contributed by the present invention pertains to apreferred method of processing such an image converter tube having a multi-layer pickup screen of the cesiated antimony type. In accordance with the invention, pickup screen'14 is entirely processed externally to the envelope enclosure of the image converter with the single exception of its antimonylayer 14d. Previous attempts to externally process pickup screens involving semi-transparent photocathodes have all resulted in failureor ina photoemissive cathode of poor quality possessing extremely poor activity.

With the reference to the flow chart in Figure 2, the

.tem and evacuated to a predetermined pressure.

It' is important that the pressure in the sub-assembly be as low as practical before heat is applied to avoid chemical interaction of the residual gas with the tube components. Therefore, in the instant case, using an envelope having a volume of approximately 15,000 cubic centimeters, the appropriate upper residual pressure limit is approximately 100 millimicrons. At step III, heat is applied to the subassembly until a temperature of approximately 350 degrees centigrade has been reached which temperature exceeds the sublimation temperature of an amorphous antimony layer. The speed of attaining this temperature is not critical. Evacuation of the envelope is continued until the pressure drops below approximately 80 millimicrons at the approximate temperature of 350 degrees centigrade. A pressure of 80 millimicrons is selected to assure a well outgassed sub-assembly. This pressure value as well as the previous pressure indicated is valid only for the tube volume being considered, that is, an image converter tube of approximately ten-inch diameter having an internal volume of approximately 15,000 cubic centimeters. Pressure limits for tubes of other volumes may be readily determined to a sufliciently close approximation by the use of the well-known ideal gas law:

Where V is volume in cubic centimeters; P is pressure in pounds per square centimeters; T is absolute temperature in degrees Kalvin; and R is a constant.

After a pressure of 80 millimicrons has been reached the oven is turned off and the tube is cooled as shown .to be step IV. When the tube has reached room temthis point is entirely conventional and includes wellknown heat treatment so that gas layers, and layers of organic material or hydroxides are eliminated. Process step Ia in the flow chart indicates that the partially completed pickup screen is baked in room air using an electric oven for a period of ten minutes at approximately 340 degrees centigrade. This temperature is selected because it is entirely compatible with the physical and chemical properties of the phosphor-resin layer 14b and the barrier layer 140 already applied to the partially processed multi-layer pickup screen. The preliminary baking of step Ia is prescribed to decompose aluminum hydroxide which might have formed on the surface of the aluminum oxide barrier layer 14c under the influence of humidity subsequent to the construction of the partially completed pickup screen.

In step Hz: the partially formed pickup screen is removed from the oven and immediately inserted into a demountable pump station while hot. It is essential that the temperature of the partially prepared pickup screen be kept above the boiling point of water or above 100 degrees centigrade while being transferred to the pump station. Evacuation is started and the antimony is evaporated onto the pickup screen barrier layer in a manner well known .to those skilled in the art. The antimony layer is evaporated from a nickel boat employing a geometry which guarantees an even thickness of antimony. This is accomplished in accordance with a technique well known to those skilled in the art. After the antimony layer has been evaporated the demountable pump station is let down to air and the bell jar thereof, containing the multi-layer pickup screen completed to the extent of having an evaporated antimony layer, is filled with bottled nitrogen using a common dryer.

In operative step V this screen is incorporated into the tube envelope. In particular, the previously baked tube sub-assembly is let down to ,air and the flanges 13a, 1312 are cut open. Preferably this is done at about the time nitrogen is tov be introduced into the demountable station. The pickup screen is now quickly transplanted and mounted in its appropriate place in the prebaked tube envelope. The transferral of the pickup screen from its demountable station into the prebaked tube envelope sub-assembly must be accomplished as quickly as possible. It is essential that exposure to room air of the prebaked tube components as well as the prepared pickup screen be as slight as possible in order to minimize the formation of undesirable oxides, especially antimony oxide.

Now the tube envelope is again sealed by heliarc welding of its flanges and as step VI shows the completely assembled tube, including the pickup screen, is placed on the processing pump station and evacuated at room temperature for as long as necessary to insure a low residual pressure during the subsequent heating cycle. When the transplant of the pickup screen into the tube envelope is made, it is important that the sealing of the two halves of the tube envelope be done in a manner in which the heat involved can be closely controlled. In the instant case the two portions of the tube envelope are sealed by heliarc welding as statedin order that the heat from the sealing operation will not cause the temperature of the pickup screen to exceed permissible limits.

After this cold pumping step, it is necessary to vacuum brake the assembly prior to the actual cesium processing of the antimony layer. The temperature to be maintained at this time must not exceed a value determined by the vapor pressure of the evaporated antimony at the residual pressure present in the tube. In the instant case, the pressure was not permitted to exceed 60 millimicrons at any time during the heating cycle. As before this value is valid only for the volume of envelope being considered, and again for volumes other than the instant case the ideal gas law may be used. With a maximum pressure of 60 mili-microns, the baking temperature of the assembled device is controlled not to exceed 240 degrees centigrade, a temperature somewhat below the sublimation temperature of the amorphous antimony layer. The prebaking process of the tube sub-assembly, as indicated in step III, results in the removal ofsubstantially all residual gases in the envelope and its sub-assembly com ponents which might adversely affect the operation of the tube. It is known thatsome gas adsorption may take place during the transplant operation of step V, but this gate is subsequently substantially removed by the low pressure baking of the complete tube assembly, and the limitation to a low pressure during this time is an additional precaution against damage to the evaporated antimony layer.

It is also entirely possible that the exposure of the pickup screen during the transplant may result in a small amount of the evaporated antimony oxidizing and transforming to antimony trioxide. Tests have determined that the sublimation point of antimony trioxide at a residual pressure of approximately 60 milimicrons is approximately 30 degrees centigrade lower than the sublimation point of the amorphous evaporated antimony itself. It has thus further been found that heating the pickup screen surface containing small amounts of antimony trioxide in vacuum at a maximum pressure of 60 millimicrons and a temperature approximately 30 degrees centigrade lower than the sublimation temperature of the amorphous antimony causes selective sublimation of the antimony trioxide and leaves the remaining amorphous antimony perfectly suitable for use in the subsequent, formation of photocathodes.

The actual baking temperature of the completely assembled tube is increased slowly, maintaining a maximum pressure of the aforementioned 6O millimicrons until the desired temperature of 240 degrees .centigrade has been reached and this temperature is held for a minimum of 30 minutes to insure removal ofgas layers ad- 7 scrbed on the internal surfaces of the assembled tube.

Thereafter, the temperature is dropped to approximately 200 degrees centigrade for a minimum of one hour after whichthe' tube is permitted to cool to room temperature and is now ready for photosurface processing.

The photosurface processing of step VII consists fundamentally of cesiating the antimony layer of the pickup screen. This is carried out in the usual manner known to pickup screen, may be carried out at high temperatures without the restriction otherwise imposed because of the presence of antimony. This is highly desirable and more completely eliminates occluded gases from the structure.

The operative steps that have been set forth avoid any objectionable oxidation of the antimony, making it possible to follow a practice in which the antimony is laid down before the screen pack is introduced into the tube envelope. This features brings about further advantages. The ability to prepare the screen pack externally permits the attainment of a more uniform antimony layer. Moreover, it makes possible the inspection of the screen pack for imperfections before introducing this component into the envelope. It is also obviousthat the external application of the antimony layer completely avoids the formation of spurious emitters characteristic of prior art procedures wherein the antimony is deposited, in situ,,within the envelope.

While a particular embodiment of the present invention has been shown and described, it is apparent that various changes and modifications may be made, and it is therefore intended in the following claims to cover all such modifications and changes as may fall within the true spirit and scope of this invention.

I claim:

1. The method of manufacturing an image converter comprising an evacuated envelope enclosing a plurality of components including a multi-layer pickup screen of the screen at an elevated temperature of not less than 100 degrees centigrade into a vacuum chamber and evaporating said antimony layer thereon; opening said envelope and inserting said partially prepared screen therein and rescaling; again evacuating said envelope and increasing the temperature to a maximum value substantially less than said sublimation temperature and at a rate to maintain the pressure in said envelope lesss than a predetermined maximum; and cesiating said antimony layer.

3. The method of manufacturing an image converter comprising an evacuated envelope enclosing a plurality of components, including a multi-layer pickup screen having a cesiated antimony layer, which method includes the steps of: making a sub-assembly by securing all compcnents of said image converter, other than said multilayer pickup screen in said envelope and sealing said envelope; evacuating said envelope to a predetermined pressure; heating said sub-assembly to a minimum temperature of approximately 350 degrees centigrade; preparing said multi-layer pickup screen complete except for its antimony layer and heating said prepared screen to a antimony-cesium type, which method includes the steps of: making a sub-assembly by securing all components of said image converter other than said multi-layer pickup screen in said envelope and sealing said envelope; evacuating said envelope to a predetermined pressure; heating said sub-assembly to a minimum temperature substantially in excess of the sublimation temperature of antimony for a predetermined period of time; preparing said multi-layer pickup screen complete except for its antimony layer and heating said prepared screen to an elevated temperature in excess of 100 degrees centigrade; introducing said heated screen into a vacuum station and evaporating said antimony layer thereon; opening said envelope and inserting said partially prepared screen therein and resealing; again evacuating said envelope and increasing the temperature to a maximum value substantially less than said sublimation temperature and at a rate to maintain the pressure in said envelope less than a predetermine maximum; and cesiating said antimony layer.

2. The method of manufacturing an image converter comprising an evacuated envelope enclosing a plurality of components, including a multi-layer pickup screen having a cesiated antimony layer, which method includes the steps of: making a sub-assembly by securing all components of said image converter, other than said multilayer pickup screen in said envelope and sealing said envelope; evacuating said envelope to a predetermined pressure; heating said sub-assembly to a minimum temperature substantially in excess of the sublimation tempera- ,ture of antimony; preparing said multi-layer pick-up screen complete except for its cesiated antimony layer and heating said prepared screen to an elevatedtemperature as high as compatible with its physical and chemical prop erties; inserting said partially prepared multi-layerpick'up temperature of approximately 340 degree centigrade; reducing the temperature of said partially prepared multi-layer pickup screen to a temperature of not less than degrees centigrade and inserting into a vacuum chamber at that temperature and evaporating said antimony layer thereon; opening said envelope and inserting said partially prepared screen therein and rescaling; again evacuating said envelope and increasing the temperature to a maximum value of approximately 240 degrees centigrade for approximately 30 minutes and at a rate to maintain the pressure in said envelope less than a predetermined maximum; and cesiating said antimony layer.

4. The method of manufacturing an image converter comprising an evacuated envelope enclosing a plurality of components, including a multi-layer pickup screen having a cesiated antimony layer, which method includes the steps of: making a sub-assembly by securing all components of said image converter, other than said multilayer pickup screen, in said envelope and sealing said envelope; evacuating said envelope to a pressure of approximately 5 X 10- millimeters of mercury or less; heating said sub-assembly to a minimum temperature substantially in excess of the sublimation temperature of said antimony layer; preparing said multi-layer pickup screen complete except for its antimony layer and heating said prepared screen to an elevated temperature as high as compatible with its physical and chemical properties; reducing the temperature of said partially prepared multilayer pickup screen to a temperature of not less than 100 degrees'centigrade and inserting into a vacuum chamber at that temperature and evaporating said antimony layer thereon; opening said envelope and inserting said partially prepared screen therein and rescaling; again evacu- "ating said envelope and increasing the temperature to a maximum value substantially less than said sublimation temperature and at a rate to maintain the pressure in said envelope less than approximately 2.5 10 millimeters of mercury; and cesiating said antimony layer.

5. The method of manufacturing an image converter comprising an evacuated envelope having a volume of approximately 15,000 cubic centimeters enclosing a plurality of components, including a multi-layer pickup screen having a cesiated antimony layer, which method includes the steps of: making a sub-assembly by securing all components of said image converter, other than said inulti-layer pickup screen, in said envelope and sealing said envelope; evacuating said envelope to a pressure of approximately 10" millimeters of mercury; heating said sub-assembly to a minimum temperature of approximately 350 degrees centigrade; preparing said multi-layer pickup screen complete except for its antimony layer and heating said prepared screen to a minimum temperature 'of approximately 340 degrees centigrade; reducing the temperature of said partially prepared multi-layer pickup screen to a temperature of not less than 100 degrees centigrade and inserting into a vacuum chamber at that temperature and evaporating said antimony layer thereon at a pressure of less than 2X10" millimeters of mercury; opening said envelope and inserting said partially prepared screen therein and resealing; again evacuating said envelope and increasing the temperature to a maximum value of approximately 240 degrees centigrade for approximately 30 minutes and at a rate to maintain the pressure in said envelope less than 6X10" millimeters 5 of mercury; and cesiating said antimony layer.

No references cited.

UNITE]? STATESPATENT OFFICE CE'TIFICATE GF GQRRECTION Patent N00 2 94l 857 June 21. 1960 Wilfrid Fa Nikias It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column l line 35}, and column 3 line 45 for "*silicoN each occurrence read silicone Signewand sealed this 29th day of May 19620 (SEAL) Attesit:

ERNEST wt. SWIDER DAVID L LADD guesting @fficer I Commissioner of, Patents 

1. THE METHOD OF MANUFACTURING AN IMAGE CONVERTER COMPRISING AN EVACUATED ENVELOPE ENCLOSING A PLURALITY OF COMPONENTGS INCLUDING A MULTI-LAYER PICKUP SCREEN OF THE ANTIMONY-CESIUM TYPE, WHICH METHOD INCLUDES THE STEPS OF: MAKING A SUB-ASSEMBLY BY SECURING ALL COMPONENTS OF SAID IMAGE CONVERTER OTHER THAN SAID MULTI-LAYER PICK-UP SCREEN IN SAID ENVELOPE AND SEALING SAID ENVELOPE, EVACUATING SAID ENVELOPE TO A PREDETERMINED PRESSURE, HEATING SAID SUB-ASSEMBLY TO A MINIMUM TEMPERATURE SUBSTANTIALLY IN EXCESS OF THE SUBLIMATION TEMPERATURE OF ANITONY FOR A PREDETERMINED PERIOD OF TIME, PREPARING SAID MULTI-LAYER PICKUP SCREEN COMPLETE EXCEPT FOR ITS ANTIMONY LAYER AND HEATING SAID PREPARED SCREEN TO AN ELEVATED TEMPERATURE IN EXCESS OF 100 DEGREES CENTIGRADE, INTRODUCING SAID HEATED SCREEN INTO A VACUUM STATION AND EVAPORATING SAID ANTIMONY LAYER THEREON, OPENING SAID ENVELOPE AND INSERTING SAID PARTIALLY PREPARED SCREEN THEREIN AND RESEALING, AGAIN EVACUATING SAID ENVELOPE AND INCREASING THE 